1. Field of the Invention
The present invention relates to an optical lens having an antireflective structure, and more specifically, to an optical lens having an antireflective structure on a lens surface.
2. Description of the Background Art
Transmissive optical elements are used for various optical systems such as those for cameras, projectors, and optical pickup devices. To obtain a predetermined optical performance, these optical systems require optical elements that efficiently transmit light. On the other hand, a technology is known of applying an antireflective coating along the surface of optical elements.
Research has been pursued on high-density and large-capacity optical disks such as DVDs (digital versatile disks) and Blue-ray disks(R). To play back information recorded on an optical disk, a laser beam is condensed on the optical disk by an objective lens, and the reflected light caused by the condensation is applied to an optical detector. To record information, a laser beam is condensed on the optical disk by the objective lens. To densely play back or record information, it is necessary that the beam spot formed on the optical disk by the objective lens be small. The beam spot is directly proportional to the wavelength of the laser beam used, and is inversely proportional to the NA (numerical aperture) of the objective lens. Therefore, a high-NA objective lens capable of reducing the beam spot is desired. For example, for DVDs, an optical lens having an NA of 0.6 is used for a wavelength of 635 nanometers, and for Blue-ray disks(R), an optical lens having an NA of 0.85 is used for a laser beam having a wavelength near 400 nanometers.
However, in the optical pickup devices, the laser beam intensity has a Gaussian intensity distribution such that the intensity is largest around the optical axis of the optical lens and gradually decreases toward the rim. In the high-NA optical lenses, since a laser beam with a short wavelength is condensed on the optical disk as mentioned above, it is necessary that the curvature of each lens surface be high. For this reason, the transmittance of the laser beam in a lens peripheral area where the incident angle is large is low. As described above, in the high-NA optical lenses, reduction in light quantity is large because of the characteristic of the light intensity distribution of the lens and the laser beam.
However, according to the method of applying an antireflective coating, since the curvature in the peripheral area of the high-NA lenses is high, the thickness of the antireflective coating applied to the peripheral area of the lens is small compared with that around the optical axis. For this reason, the laser beam transmittance in the lens peripheral area is low and it is therefore difficult to reduce the beam spot diameter.
On the contrary, Japanese Laid-Open Patent Publication No. 2004-145003 (hereinafter, referred to as Patent Document 1) proposes a technology associated with a method of excellently applying an antireflective coating to the high-NA optical lenses. Patent Document 1 proposes an optimum coating thickness design condition based on the angle of the incident light.
However, the antireflective coating method described in Patent Document 1 can be realized by controlling the minute coating thickness. For this reason, it is difficult to provide a high-NA optical lens to which an appropriate coating is actually applied according to the design condition. In addition, the applied antireflective coating has a large thickness error from the design thickness, so that it is extremely difficult to obtain an optical lens having excellent transmittance.
In the optical pickup devices, to obtain excellent recoding/playback signals, it is desirable for the optical lens to have a uniform light quantity distribution from the axial to peripheral areas. However, the thickness error from the design thickness increases as the curvature of the lens surface increases. For this reason, in the high-NA optical lenses, the thickness error in the lens peripheral area is large, so that it is difficult for the light quantity distribution to be uniform. Consequently, the overall transmittance of the optical lens is low, so that it is difficult to form an excellent beam spot on the optical disk. As described above, it is difficult to provide a high-NA optical lens provided with excellent transmittance by the application of an antireflective coating.